1. Field of the Invention
The present invention relates to an optical scanning device in an image forming apparatus, and a control method thereof and, more particularly, to an optical scanning device in an image forming apparatus such as a copying machine or printer, a control method thereof, and the image forming apparatus.
2. Description of the Related Art
Recently, copying machines and printers which form an image using laser scanning are requested to have higher speeds and higher resolutions. To meet this request, there are proposed a technique of rotating at a higher speed a polygon mirror which scans a laser, and a technique of increasing the number of emitting portions from which laser beams are emitted.
However, speeding up of the polygon mirror has limits considering heat and noise generated by windage, the strength of the mirror itself, the rotation capability of the motor, and the like.
From this, the mainstream for increasing the speed and resolution is shifting to the use of light of multi-beams emitted from a high-density light source such as a vertical-cavity surface-emitting laser (VCSEL). This is a technique of increasing the number of laser beam-emitting portions much more than the conventional one by using a high-density light source, and scanning with many laser beams in parallel.
To increase the speed, an image forming apparatus of a so-called tandem system is advantageous among recently popular color image forming apparatuses. In the tandem system, images of four colors, that is, yellow, magenta, cyan, and black are formed on independent photosensitive members, and finally overlaid on a transfer member. The image forming apparatus of the tandem system obtains a color image by overlaying images formed by separate scanning mechanisms. If the inclination or curvature of each scanning line occurs in the sub-scanning direction on the photosensitive member, this appears as color misregistration or color inconsistency and becomes a factor of making the image quality lost.
There has been conventionally known a method for correcting the curvature in the sub-scanning direction of a scanning line. For example, there is a method of canceling the curvature by mechanically bending the reversion mirror of an optical scanning device. Japanese Patent No. 3600228 discloses another method of correcting the curvature in the sub-scanning direction of a scanning line. According to this method, an image formable effective scanning area is divided into a plurality of regions in the main scanning direction. The write position in each divided region is offset in the sub-scanning direction, correcting the curvature in the sub-scanning direction of a scanning line.
However, the following problems arise in the conventional methods of correcting the curvature in the sub-scanning direction of a scanning line.
It is technically possible to increase the number of emitting portions to several ten in an optical scanning device using a VCSEL or the like in order to meet the recent demand for high speeds and high resolutions. If the number of laser beams reaches several ten, the laser beams distribute widely in the sub-scanning direction. Assuming that the number of scanning lines is 48 in a 2,400 dpi-resolution optical scanning device, the distance between scanning lines at both ends in the sub-scanning direction increases to about 0.4 mm.
A main factor of the curvature in the sub-scanning direction of a scanning line is aberration of an imaging lens. The aberration changes depending on the sub-scanning position of a laser beam passing through the imaging lens, so the value of generated curvature changes depending on the laser beam. In some cases, the difference in curvature value between laser beams spaced apart from as much as 0.4 mm increases to 5 to 10 μm. In an image forming apparatus, even 5-μm color misregistration lowers the image quality, so the difference in curvature value should be eliminated.
To the contrary, according to the conventional mechanical mirror bending and the method of correcting the curvature in the sub-scanning direction of a scanning line as disclosed in Japanese Patent No. 3600228, all laser beams are uniformly corrected, and the difference of curvature value in the sub-scanning direction between scanning lines cannot be eliminated.